The parents of 23 babies are awaiting the results of tests to determine whether their children are infected with the same bacterium that has killed three infants at a Belfast hospital.

The Belfast Health and Social Care trust confirmed that a third baby had died in the neonatal unit of the city's Royal maternity hospital after an outbreak of Pseudomonas aeruginosa.

On Thursday, the trust said two other babies in the same unit had died from the infection since 6 January. They had been born prematurely.

Dr Clifford Mayes, a consultant neonatologist, speaking alongside the Northern Ireland health minister, Edwin Poots, at a press conference in Belfast, said staff at the Royal had first become aware of the problem on Monday night.

Mayes said: "It takes 48 hours before an infection can be positively identified. The clinical reality is that you take blood samples … These are sent to the laboratory, so you don't know at the time of death what the cause is. We may learn that these deaths are from different strains – there is still a lot of information to come back."

Clinicians and health officials said they were considering moving some babies out of the unit. Colm Donaghy, chief executive of the Belfast trust, said: "We have access to neonatal units across Northern Ireland. The other trusts are co-operating with us. There is enough capacity at this time. We have arrangements to deal with peaks."

Poots, a Democratic Unionist party minister, expressed his sympathies to the families of the three babies who had died. He said: "I have asked the trust to work with the Public Health Agency to ensure all necessary steps are swiftly taken to identify the source of the infection so that we contain it and reduce further risk of spreading."

Pseudomonas bacteria, which can cause severe breathing and other problems, are found in soil, water, plants and animals and are particularly harmful to patients who are already ill, such as the elderly and small babies with weak immune systems.

Commenting on the infection, Dr Katie Laird, a senior lecturer in pharmaceutical microbiology at De Montfort University, Leicester, said: "Pseudomonas is an opportunistic bacterium that tends to affect those with weaker immune systems.

"It can cause a range of symptoms from urinary tract infections to respiratory complications and in recent years has become increasingly resistant to some antibiotics."

• This article was amended on 31 January 2012. The original referred to Pseudomonas aeruginosavirus. This has been corrected.